Methods for coloring and decolorizing transparent glass-ceramics have been the subject of numerous patents. Several examples of such are set out below:
U.S. Pat. No. 3,788,865 (French Pat. No. 1,474,728) recommends the use of the transition metal oxides CoO, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, MnO, and V.sub.2 O.sub.5 to tint transparent glass-ceramics. It appears that those oxides act either individually or in combination with one another or, also, in combination with other known colorants such as copper sulfide and copper oxide. The tints that could be obtained were designated as ruby, pink, yellow, amber, blue, lavender, and purple.
Netherlands Pat. No. 66 05388 provides another description of the coloring of transparent glass-ceramics, making use of CoO, Cr.sub.2 O.sub.3, CuO, Fe.sub.2 O.sub.3, MnO.sub.2, NiO, WO.sub.3, CdS, CdSe, SeS, and SeS.sub.2 as colorants and reports the following colors as being attainable: red, purple, orange, dark blue, green, brown, and gray.
Other patents such as U.S. Pat. Nos. 4,009,042 and 4,018,612 mention the utility of transition metal oxides as colorants for transparent glass-ceramics, but without specifying the colors obtained nor, moreover, the method to achieve those colors.
U.S. Pat. No. 4,211,820 teaches how to confer a red-brown coloration to transparent glass-ceramics which is sufficiently pronounced to reduce to about 20-60% the transmission factor at 800 nm of a sheet about 5 mm thick. The products have a base composition in the general Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 field nucleated with TiO.sub.2. The desired coloration is developed by the inclusion of V.sub.2 O.sub.5 in the starting batch.
Corning France S.A., Le Vesinet, France, commercially markets under the trademark VISION cooking utensils of a transparent ceramic exhibiting a light brown tint prepared as described in French Pat. No. 2,405,906; viz., the base compositions--generally in the system Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 nucleated with TiO.sub.2 --form a body displaying an amber color because of the presence of TiO.sub.2 as the nucleating agent and the presence of iron oxides as impurities in the batch materials. In order to end up with the desired light brown color, suitably proportioned quantities of Nd.sub.2 O.sub.3 and V.sub.2 O.sub.5 are added.
The selected color is characterized by its trichromatic coordinates (x, y, Y) determined in accordance with the procedure standardized by the C.I.E.: a sheet having a thickness of 4 mm with polished surfaces of a reference light brown glass-ceramic, when illuminated with Illuminant C (x=0.310, y=0.316) exhibits the coordinates x=0.331, y=0.337, and the visual transmission factor Y=67%. Other tints with trichromatic coordinates close to those of the reference can be utilized so long as the human eye does not perceive any significant difference, which circumstance holds true so long as the dominant wavelength to which the article is exposed remains between about 577-579 nm and the purity between 8-14% (see control graph in FIG. 1).
One disadvantage in the use of that process for obtaining the light brown color is the high cost of Nd.sub.2 O.sub.3, which is produced in such limited quantities that slight fluctuations in demand can have a devastating effect upon prices.